A typical gas turbine that is used to generate electrical power includes an axial compressor at the front, one or more combustors downstream from the compressor, and a turbine at the rear. Ambient air may be supplied to the compressor, and rotating blades and stationary vanes in the compressor progressively impart kinetic energy to the working fluid (air) to produce a compressed working fluid at a highly energized state. The compressed working fluid exits the compressor and flows towards a head end of combustor where it reverses direction at an end cover and flows through the one or more nozzles into a primary combustion zone that is defined within a combustion chamber in each combustor. The compressed working fluid mixes with fuel in the one or more fuel nozzles and/or within the combustion chamber and ignites to generate combustion gases having a high temperature and pressure. The combustion gases expand in the turbine to produce work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity.
In a particular combustor design, one or more fuel injectors, also known as late lean fuel injectors, are circumferentially arranged around the combustion chamber downstream from the nozzles and/or the primary combustion zone. A portion of the compressed working fluid exiting the compressor is routed through the fuel injectors to mix with fuel to produce a lean fuel-air mixture. The lean fuel-air mixture may then be injected into the combustion chamber for additional combustion in a secondary combustion zone to raise the combustion gas temperature and increase the thermodynamic efficiency of the combustor. The late lean fuel injectors are effective at increasing combustion gas temperatures without producing a corresponding increase in the production of undesirable emissions such as oxides of nitrogen (NOx). The late lean fuel injectors are particularly beneficial for reducing NOx during base load and/or turndown operation of the gas turbine. In contrast, during certain non-base load operation modes such as during start-up, cold fuel and liquid fuel operation late lean fuel injection is undesirable, thus the late lean fuel injectors are not fueled.
Although fuel to the late lean fuel injectors may be shut off during operation of the gas turbine, the compressed working fluid flowing to the late lean fuel injectors is routed through a passive circuit that is defined within an outer casing such as a compressor discharge casing and thus cannot be shut off. As a result, the compressed working fluid flows through the late lean fuel injectors and the liner and mixes with the combustion gases flowing through the hot gas path, thereby causing air dilution of the combustion gases which may result in undesirable emissions levels. To overcome the effects of the air dilution, an operator must over fire the one or more fuel nozzles that feed the primary combustion zone. However, over firing may result in high combustion liner and/or transition duct wall temperatures which limits the mechanical life of those hot gas path components. Therefore, a system for controlling a flow rate of the compressed working fluid to the fuel injectors would be useful.